FLORIDA STATE HORTICULTURAL SOCIETY 
65 
solution. In other words, that amount of 
P. A. is supposed to be available which 
dissolves in this solution. The solution is 
supposed to approximate the soil solution, 
but of course it is a purely arbitrary stan¬ 
dard, as indeed any test must be. For 
soil conditions are so variable and so many 
different factors enter into consideration 
that no accurate measure of availability 
can be provided. Soil chemists do not 
maintain that the available P. A. is accu¬ 
rately measured by this test. It is said 
that they are now working to perfect the 
better standard. As a matter of fact, 
much more P. A. is available to healthy, 
growing trees under the best soil condi¬ 
tions and management than would be 
shown available by any chemical test. Liv¬ 
ing organisms have selective and solvent 
powers of their own. It is asserted that 
the available P. A. content of slag should 
be determined by its solubility in a 2 per 
cent citric acid solution—the Wagner test 
—since its alkalinity reduces the solvent 
action of the legal solution. This Wag¬ 
ner test gives slag 16 per cent available 
P. A., while the Florida test will show 
not more than 5 per cent. 
Slag and Acid Phosphate. Availabil¬ 
ity of slag is said to be due to the disinte- 
grating effect of the slaking of the CaO 
associated with the tetracalcium phos¬ 
phate. The slaking might be completed 
between the times of manufacture and ap¬ 
plication. This calcium oxide is variously 
estimated at from 2 to 5 per cent—a very 
slight amount compared to the needs of 
Florida soils. The carbonate, says Van 
Slyke, is insignificant. No other forms of 
calcium except CaO, CaHO, CaC02 are 
of any value in correcting existent acidity 
and cannot properly be called lime. The 
burden of evidence is that slag does not 
contain at the outside more than 5 per 
cent of genuine lime. Certainly it is of 
value as furnishing an alkaline source of 
P. However, as a dependent source of 
lime, it is both expensive and insignificant. 
If the total P. A. be worth $1.00 a unit 
the lime, iron and magnesia may be con¬ 
sidered extras. The claims now being 
made for slag seem extravagant. Take 
this matter of 14 per cent iron content. 
While iron is found in connection with 
chlorophyl in most plants, it is in such in¬ 
finitesimal quantities and the amount in 
the soil is so proportionately great that 
there is no need of applying any. Cer¬ 
tainly the presence of unusual amounts of 
iron will tend to lock up soluble P. A. as 
the insoluble phosphate of iron. 
A. P. ranks first in availability. Fresh 
A. P. is easily soluble in soil waters, and 
is thus widely diffused before reversion. 
A considerable quantity of the monocal¬ 
cium will revert in the presence of calcium 
to the less soluble dicalcium (1-18) as 
soluble. The reversion may even con¬ 
tinue to tricalcium phosphate, which is or¬ 
dinarily not considered available. How¬ 
ever, the solubility of Di and Tricalcium 
is much increased by the carbon dioxide 
universally present and some common 
forms of fertilizer as nitrate of soda, sul¬ 
fate of ammonia and sulfate of potash. 
Unfortunately, it is retarded by iron com¬ 
pounds and gypsum. When we consider 
that A. P. contains more gypsum than 
anything else, we cannot help but consider 
so large a quantity rather objectionable. 
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